High speed data transmission has resulted in the development of a number of types of printing devices that can accept input data at a high rate. U.S. Pat. No. 3,803,558, which is assigned to the same assignee as this invention, discloses one such system wherein the printing apparatus utilizes a continuously moving character belt that carries the type faces for each character to be printed. A plurality of hammers are arranged in a row across the face of a record medium such as paper, the position of each hammer establishing a column in which a character may be printed. Typically, an inking ribbon is positioned in front of the record medium and the path of the character belt is located behind the inking ribbon and in front of the hammers.
It is essential that the printer be capable of printing any desired character in any position across the record medium. Accordingly, means are provided for discretely indicating to control circuitry, where each character appears relative to the record medium. When this is established, circuitry is provided for energizing the hammers at an appropriate position to imprint the desired characters.
As disclosed in the aforecited patent, a typical character belt carries a number of flexible fingers, each of which has the type face for a particular character on one end thereof. The type carrying belt is mounted upon drive devices which rotate about parallel axes in order to effect passage of the fingers between the recording medium and hammers. The position of each character relative to a fixed point on the printer, is determined by detecting the passage of the flexible fingers past this fixed point. Generally, photoelectric means are positioned to effect this detection and generate a representative electric signal. The essence of the present invention is to establish accurately the position of each type finger relative to the hammers and record medium.